Although most patients with Stickler syndrome were found to have the type 1 vitreous pheno-type, it became clear that a minority of pedigrees had the same classical systemic features and the same risk of retinal detachment but had a different vitreous phenotype. The vitreous was also highly abnormal but instead of the classical membranous anomaly, sparse, irregularly thickened, "beaded" fibrils were seen throughout an otherwise empty-looking gel (type 2 vitreous phenotype). Linkage to COL2A1 was excluded. Mutations in the gene encoding the ai chain of type XI collagen (COL11A1) on chromosome 1 have been found in seven families [23, 31, 40] and these are, to date, the only mutations associated with the type 2 vitreous phenotype. These pedigrees have a similarly high risk of detachment and giant retinal tear but appear to have a higher prevalence of sensorineural deafness than the type 1 Stickler families .
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